a. Field of the Invention
The present invention is directed to an improved process for producing a viscosifying carboxylic acid copolymer. The polymer produced by the process of the present invention has a viscosity that is equal to or greater than 50,000 centipoise ("cps") when measured in water at a 0.5% by weight concentration. The process of the present invention is useful because it is capable of producing a high viscosity copolymer that is suited for use in foods, cosmetics, printing inks, pastes, and coating applications.
b. Background
Numerous processes are known in the art for producing viscosifying polymers. However, each of these processes suffers from one or more infirmities. For example, EPO Publication No. 0 371 421 A2, which was published on Jun. 6, 1990, discloses a process for producing a polycarbophyl or cross-linked polyacrylic acid polymer in a solvent selected from acetone, alkyl acetates, and mixtures thereof. In particular, the '421 publication discloses only a single polymer having a viscosity for a 1% solution that exceeded a 50,000 cps at pH 7.2 to 7.8 at 20 rpm. One problem with the '421 process is that it uses the organic solvent ethylacetate, or acetone, either of which is flammable, hazardous, and requires special handling provisions. A second problem with the process of the '421 disclosure is that the best polymer that was capable of being produced by the disclosed process only had a viscosity of 59,200 cps for a 1% solution.
U.S. Pat. No. 3,915,921, which issued to Schlatzer on Oct. 28, 1975, discloses a viscosifying polymer that is produced by copolymerizing a carboxylic acid monomer and one or more alkyl acrylate esters. The '921 patent, like the previously discussed '421 patent, discloses copolymerization that occurs in an organic solvent such as benzene, xylene, tetralin, heptane, hexane, carbon tetrachloride, methylchloride, ethylchloride, bromotrichloromethane, dimethylcarbonate, diethylcarbonate, ethylenedichloride, and mixtures thereof. Thus, a problem with the '921 process, like that of the '421 process, is that it utilizes hydrocarbon solvents, which are flammable and in many instances hazardous to health, while the halocarbon solvents are generally just hazardous. Polymerization in any one of the disclosed solvents can give rise to hazardous and/or flammable vapor emissions, which require special precautions. A second problem with the process disclosed in the '921 patent is that the best viscosity that was reported for any polymer at the 0.5% by weight concentration in water was 71,200 cps.
U.S. Pat. No. 4,509,949 to Huang, which issued on Apr. 9, 1985, discloses a process for producing water thickening agents consisting of copolymers of acrylic acids and esters that are cross-linked with a poly-functional vinylidene monomer containing at least two terminal CH.sub.2 groups. One problem with the '949 process, like most of the processes of the prior art, is that the '949 process also teaches polymerization in a generally hazardous solvent such as "benzene, tetralin, hexane, heptane, cyclohexane, carbontetrachloride, chloroform, trichloroethylene, methylchloride, ethylchloride, and methylenechloride; chlorofluoroalkanes, such as chlorofluoromethane and chlorofluoroethane, each containing at least four halogen atoms; esters such as methylacetate and ethylacetate, alcohols including methanol, ethanol, butanol and the like." ['949 at col. 4, Ins. 37-44.] Another problem with the '949 process is that it is only capable of producing polymers that at best provide modest increases in viscosity. For example, the best viscosity that was produced by the products of the '949 process was 12,000 cps for a 1.2% by weight solution of the polymer. ['949 at col. 8, ln. 8.]
In view of the problems commonly associated with the '421, '921 and '949 patents, it is a first object of the present invention to produce a viscosifying polymer in a non-hazardous and a non-flammable solvent that generally does not require special handing. In addition, it is an object of the present invention that the polymer produced by the process of the present invention have a viscosity of at least 50,000 cps when measured as a 0.5% by weight solution in water at room temperature.
One method for producing a viscosifying polymer that does not use hydrocarbon or halocarbon solvents is disclosed in EPO Publication No. 0 301 532 A2, which was published on Feb. 1, 1989. The '532 disclosure teaches that copolymerization can be effected in carbon dioxide to yield a fluffy powder when the acrylic acid, the comonomer, and the chemical initiator are dissolved in a single liquid phase, i.e., liquid carbon dioxide. The best viscosity that was obtained with the product produced by the '532 process was 12,550 cps for 0.2 weight % solution in water. One problem with the '532 process is that the carbon dioxide must be pressurized in order to form a liquid at the reaction temperatures. In particular, pressures of 1200 to 2500 pounds per square inch would not be uncommon. Such pressurized reactions require special reaction vessels and equipment. An object of the present invention is to provide a process for producing a viscosifying polymer that is capable of utilizing a conventional unpressurized reactor.
A second problem with the '532 process is that it produces a product having a low viscosity (12,550 cps for a 0.2 weight % solution). To overcome such low viscosities, an object of the present invention is to provide a process that produces a viscosifying copolymer that exhibits a viscosity of 50,000 cps or greater in a 0.5% by weight aqueous solution.